1. Field of the Invention
This invention relates to powdery and granular material treating technique, and more particularly to technique effectively applicable to coating of pharmaceutical preparations, food product and the like.
2. Related Art Statement
In general, the coating apparatuses of this type include ones wherein a plurality of ventilating portions are provided in the circumferential direction of a rotary drum called a coating pan, and drying air or the like is fed to or exhausted from the interior of the rotary drum through at least one of these ventilating portions and ducts communicated therewith by an air switching valve. An apparatus of this type is disclosed, for example, in U.S. Pat. No. 3,834,347.
On the other hand, as the coating apparatuses of another type, there are ones wherein a multiplicity of ventilating perforations are provided on the entire circumference of the outer periphery of the rotary drum and seals being in sliding contact with the outer periphery of the drum are also provided, whereby drying air or the like is fed into or exhausted from the rotary drum through these ventilating perforations. An apparatus of this type is disclosed, for example, in U.S. Pat. No. 3,573,966.
Now, according to the former technique, the area of the ventilating portions is restricted to some degree. Hence, in order to increase the drying capacity, it is necessary to increase the capacity of a ventilating means. Furthermore, this may result in making the construction of the ventilating portions and the air switching valve somewhat complicated.
Whereas, in case of the latter technique, the ventilating perforations are formed on the entire outer periphery, so that the ventilating capacity can be high, a satisfactory drying capacity can be obtained, the apparatus can be simplified in construction and costs can be reduced. However, in manufacturing the apparatus, punching process is used for forming the ventilating perforations, whereby burrs are formed on the surfaces of the drum during the punching. Because of this, when the burrs are formed on the inner surface of the drum, the material tumbling in the drum, such as tablets, are damaged, thus deteriorating the product quality. On the contrary, when the burrs are formed on the outer surface of the drum, a disadvantage is presented that the seals disposed at feeding-exhausting portions are damaged to cause gas leakage. Furthermore, when the burrs on the ventilating perforations are to be removed one by one, a disadvantage is presented that a tremendous labor and time are needed for reamer finishing of the multiplicity of ventilating perforations and the costs are raised as well.
The inventors of the present invention have found the above-described disadvantages as the result of studying the aforesaid techniques.